The Twenty-one, or 3TA, introduced in 1957, marked a significant step in the evolution of the Triumph twin, bringing unit construction to the range. The new engine followed established Triumph design practices except for the use of a distributor mounted on top of the crankcases behind the cylinders. Although the use of unit construction was of greater significance in the long term, it was the model's styling that created the most interest at the time of its release. The machine was equipped with a fully valanced front mudguard complimented by a rear enclosure that was quickly dubbed the "bathtub". The new 350cc was rapidly followed by a 500cc version at the end of 1958, typed the 5TA, which replaced the pre-unit 5T Speed Twin and continued the name. The new machine shared the 350cc version's 65.5 mm stroke but adopted a bore of 69mm to give a capacity 490cc, in virtually all other respects, other than colour, the new Speed Twin was the same as its smaller brother resulting in a compact easy handling tourer with good performance that quickly found favour.
The example offered is presented to a very high standard following a well executed restoration. It has been converted to twelve volt electrics and has had only two owners from new. We are informed by the owner that it runs well and it is accompanied by a Swansea V5C.